Menthyl lactate (ML) is used in many consumer products for which a long-lasting physiological cooling effect is desired. Such products include comestibles, toiletries, pharmaceuticals, and cosmetics. Pure ML has a faint, minty odor and is practically tasteless.
Despite its popularity in consumer products, menthyl lactate lacks adequate storage stability. ML is commercially available as a solidified distillate or as a crystalline product. Upon storage, usually within weeks, the melting point drops significantly, the product develops an acidic, pungent odor, and the ML consequently becomes unusable for most intended applications. Moreover, the solid product is often melted several times before its incorporation in the consumer product. This “thermal loading” further reduces quality, which is frequently accompanied by a rise in acid number.
U.S. Pat. No. 5,783,725 teaches to prevent the acidic, pungent note by stabilizing ML with an alkali metal and/or alkaline earth metal carbonate and/or bicarbonate, which can be added during crystallization. Normally, the crystallization is performed in an organic solvent (e.g., acetone) in the presence of the inorganic salt (see Example, col. 2, II. 29-41). Stabilizing ML with the inorganic salt overcomes the need to formulate immediately after purchase. However, the mixture of ML and inorganic salt obtainable by this method has its own drawbacks, particularly when the salt is less compatible than ML with other components used to make the consumer product. The inorganic salt can confound formulators by triggering undesirable and often unpredictable phase separation.
U.S. Pat. Appl. Publ. No. 2006/0165783 teaches that menthyl lactate with improved stability can be obtained by compacting solid (flaked or crystalline) ML, typically under 30-80 kN of force, into pellets, spheres, or other geometric forms. Such high-pressure equipment is preferably avoided, however. Moreover, compacted ML is less convenient to use compared with, for example, crystals or a free-flowing powder.
U.S. Pat. No. 7,173,146 teaches a process for making menthyl lactate. Menthol and lactic acid react in the presence of a base to make ML and higher lactoyl esters of ML, followed by controlled hydrolysis to convert the higher lactoyl esters back to ML. The reference teaches that the “esterified product containing ML and higher lactoyl esters can be purified, if desired, by any suitable means, including distillation, crystallization, or the like, but it is preferably used ‘as is’ for the next step, which involves controlled hydrolysis” (see col. 4., II. 16-23). The '146 patent later teaches that after hydrolysis, the “ML product can be purified by any suitable method, including, for example, distillation, crystallization, precipitation, sublimation, or a combination thereof. Distillation is preferred” (col. 5, II. 19-26). All of the examples of the '146 patent defer purification until after controlled hydrolysis, and all of the examples use vacuum distillation to purify the ML. Thus, the '146 patent teaches a variety of purification schemes, with distillation preferred, and fails to disclose the specific combination of water and a solution of ML in a water-miscible organic solvent to precipitate ML. Moreover, nothing in the reference suggests the desirability of using aqueous precipitation instead of, e.g., distillation or crystallization, to enhance the stability of ML.
In sum, a need remains for a way to make menthyl lactate having improved storage stability. Preferably, the method would provide an easy-to-handle powder or crystalline ML rather than large pellets or compacted material and would avoid the need for high-pressure equipment. A valuable method would improve ML stability without the need for the inorganic salt stabilizers that can interfere with compatibility. An ideal method would be easy to practice with conventional equipment and reagents.